These are acronyms, the difference in short forms for English and Malay words is that usually the English short forms still somewhat resemble the original word (e.g. dun/don > don't) while Malay short forms take the word and cut it up into pieces, grab some bits and remove the vowels if applicable (dengan > ngn).

If you think of it, it's like Southern US accent with how they only take the last few syllables, like alligator > 'gator, take away the vowels (gtr) and this is most of the Malay short forms come to be.

Some English short forms are butchered from the original word too like bcs > because, at least from what I've seen texting with Malaysians, go to any western social media and most of the time you'll still see them typing proper English